THE GIST

- Armored shark-like extinct fishes were probably the first animals to have had intimate sex by copulation.

- The world's first jaws likely evolved to facilitate mating, and not feeding, as had been previously thought.

- Genes responsible for making our limbs, and the pelvic fins of fishes, probably also played a role in developing the first sexual organs.

Fish were the first to have intimate sex, suggests new research based on well-preserved fossils of extinct armored fishes from the Gogo Formation of Western Australia.

Intimate sex by copulation likely first happened in the early Devonian Period around 400 to 410 million years ago, scientists say.

This was "not just spawning in water, but sex that was fun," according to project leader John Long, who also thinks that jaws evolved in conjunction with mating.

"Jaws might not have first evolved for feeding, as widely presupposed, but to facilitate copulatory mating," said Long, who is vice president of research and collections at the Natural History Museum of Los Angeles County. "In many sharks the jaws are used to hold on to pectoral fins of females so copulation can take place."

The scientists studied fossil embryos -- still in good condition -- from Late Devonian ptyctodontid and arthrodiran placoderm fishes, which were similar to modern sharks. While these remains date to 380 million years ago, the overall fossil record for the fishes suggests they evolved the ability to have intimate sex by copulation at an earlier period.

Long and his colleagues made headlines a few years ago when it was announced that one of the fishes, an individual from the genus Materpiscis, was the world's first mother, since a female's fossil still retained a single embryo connected by an umbilical cord. The discovery represents the oldest evidence of an animal giving birth to live young.

"Our finds show that these extinct armored fishes, the placoderms, had intimate copulation with males inserting claspers (a structure that is part of the pelvic fin) inside the female to deposit sperm," Long told Discovery News.

The find "is significant because it means that an advanced form of reproduction involving copulation and live-bearing was more widespread than previously thought."

Recent genetic analysis of the fish fossils indicates that the same genes responsible for making our limbs, and the pelvic fins of fishes, also played a role in developing the sexual organs of our very distant ancestors. These genes also control development of fish jaws. The origin of the pelvic girdle of the ancient fish and the origin of their jaws even seems to have happened around the same time, based on the genetic evidence.

"It seems that limbs and genitals developed via the same developmental pathways, so fossils showing the oldest evidence of pelvic fins (the placoderms) also showing the oldest expression of sexual organs (claspers) might not be such a coincidence," Long explained.

Long and his team announced the determinations today at the Society of Vertebrate Paleontology's 70th Anniversary Meeting in Pittsburgh.

Steven Salisbury, a lecturer in the School of Integrative Biology at the University of Queensland, believes the ancient fish fossils, including the fossilized embryo, are a "spectacular" discovery that have "important implications for our understanding of the evolution of live-bearing in vertebrates.

Make love not war may even help to explain the origin of intimate sex since Salisbury added, "The large size of the embryo relative to the mother indicates that the young of this fish were born well-formed, a strategy that may have evolved to counter predation from other larger fishes."